This invention relates to a method of printing and more particularly to a method of printing utilising a print head of the so called ink jet kind, although the invention may be applied to a method of printing having other kinds of print head comprising a plurality of printing elements which may be actuated independently to print on a substrate.
In one kind of ink jet printer, there is provided an array of printing elements e.g. arranged in a linear or substantially linear array, and each of which may create a jet of ink and propel the ink towards a substrate. The jets may be created by, for example, passages in the print head which are caused to constrict by piezzo electric means, to squeeze ink therefrom or by other similar means.
The design and manufacturing tolerances of such print heads is demanding and it will be appreciated by those skilled in the art, that the performance of the individual printing elements will in practice vary, such that print heads tend to be imperfect. This variation in performance of the printing elements leads to print image irregularities. Where the variation in performance of the individual printing elements is too great, the print head has to be rejected.
The variation in performance may mean that the amount of ink and/or the velocity of the ink in the created jet is not constant for all of the printing elements. The latter of these produces the result that the ink in one or more jets reaches the substrate at a different time to ink from others of the jets, even where the jets travel substantially the same distance from the print head to the substrate. In general, in a printer of this kind, during printing there is relative movement between the substrate and the print head, and consequently, the quality of the image is impaired by an amount proportional to the velocity variation.